Method of using structural adhesive

Abstract

The disclosed method for forming an epoxy adhesive-bonded joint between a plurality of substrates is particularly well suited to an assembly line operation. The method involves forming an intimate mixture of the two components of a two-part adhesive system under conditions such that substantial curing will not occur until heat and/or pressure are applied. The application of heat and/or pressure for a short period of time initiates the curing reaction which will continue to completion under substantially normal ambient conditions. The initial bond strength is sufficient to hold mated substrates together with the bond strength increasing even when the curing process is completed under normal ambient conditions. For example, substrates can be coated with an intimate mixture of a two-part system suitable for use in this invention, stored for a period of time, mated on an assembly line under cure initiation conditions, and allowed to cure at normal room temperature and atmospheric pressure. Generally, conventional epoxy resins known in the art will work in the disclosed invention. The active hydrogen-containing curing agents or coreactants used in the disclosed invention are primary-amine terminated polyamides having a ball and ring softening point of 60°-200° C.

What is claimed is: 
     
       1. A one-part, curable epoxy prepolymer/polyamide system comprising: a. a continuous phase comprising a polar solvent system containing a polar organic liquid, said solvent system being generally incapable of dissolving an epoxy prepolymer, said polar organic liquid having a boiling point below 200° C. under normal ambient conditions; and, dissolved in each 100 parts by weight of said solvent system, 5 to 300 parts by weight of a polyamide having a ball and ring softening point within the range of 60°-200° C. and containing at least 20 mole % terminal groups which are residues of 1,4-bis-alkylamino piperazine, and, linked by amido linkages thereto, the residue of a polyfunctional carboxylic acid; and   b. a discontinuous, substantially non-settling phase dispersed uniformly throughout said continuous phase, said discontinuous phase being selected from the group consisting of solid particles of a solid epoxy prepolymer and droplets of a liquid epoxy prepolymer, said epoxy prepolymer having an epoxide equivalent weight greater than 60.   
     
     
       2. A one-part composition according to claim 1 wherein said solvent system comprises a mixture of a lower alkanol, water, and a lower alkanoic acid; and wherein said epoxy prepolymer is a finely-divided solid which is substantially insoluble in said solvent system. 
     
     
       3. A coated substrate comprising a solid substantially uncured, substantially solvent-free coating on said substrate, said coating comprising the composition of claim 1 wherein said solvent system has been removed by evaporation. 
     
     
       4. A coated substrate according to claim 3 which has been heated to at least 60° C. to cure said coating. 
     
     
       5. A method for forming an epoxy adhesive-bonded joint between a plurality of articles coated with both parts of a two-part epoxy adhesive, said method comprising: a. blending said two parts of said epoxy adhesive, one of said parts comprising an epoxide prepolymer dissolved in solvent, the other of said parts comprising a solid polyamide having a softening point within the range of 65°-200° C. and at least 20 mole % terminal reactive primary amine groups which are residues of 1,4-bisalkylamino piperazine, said polyamide being dissolved in a solvent, the solvents for said epoxide prepolymer and said polyamide being selected to provide a cosolvent system for both parts of said epoxy adhesive;   b. coating the resulting blended, two-part epoxy adhesive onto a surface of a first said article to form a wet film;   c. evaporating off said cosolvent system to provide a dry film of two-part epoxy adhesive, essentially before the dissolved polyamide and epoxide prepolymer have an opportunity to chemically interact;   d. mating a second said article with the coated article with the coated article obtained in said steps (b) and (c), under heat and pressure, whereby curing of said two-part epoxy adhesive is initiated; and   e. removing said heat and pressure and continuing said curing under normal ambient conditions.   
     
     
       6. A method for forming an epoxy adhesive-bonded joint between a plurality of substrates, comprising: a. applying to at least one of said substrates a substantially uniform mixture comprising: 1. a solution comprising a solid, primary-amine terminated polyamide dissolved therein and a suitable solvent therefor, said polyamide having a ball and ring softening point within the range of 60°-200° C.;   2. uniformly and intimately distributed through said solution, an epoxide prepolymer having an epoxide equivalent weight greater than 60; said epoxide prepolymer being substantially unreacted with said polyamide;     b. removing sufficient solvent from said solution to cause said uniform mixture to become a non-flowable material under normal ambient temperature and pressure conditions; said non-flowable material being capable of cure initiation at temperatures above 60° C. and being capable of continued curing at normal ambient temperatures, subsequent to such cure initiation;   c. mating said plurality of substrates;   d. forming a joint with the thus-mated substrates under epoxide cure initiation conditions, which conditions include applying a temperature greater than 60° C. to said joint for more than 1 second but less than 20 minutes, whereby said polyamide is:   
     
     
       1. brought to a temperature above its ball and ring softening point; 2. forms a flowable phase reactive with said epoxide prepolymer; and   3. enters into an amine/epoxide curing reaction with said polyamide;   e. removing said epoxide cure initiation conditions before said amine/epoxide curing reaction is complete, but after the strength of the joint is sufficient to permit further handling of the mated substrates;   f. continuing said amine/epoxide curing reaction under substantially normal ambient pressure and temperature conditions for at least 12 hours, the continuing of said curing reaction being characterized by an increase in the tensile shear strength of said joint.   
     
     
       7. A method according to claim 6 wherein said forming step includes applying pressure and temperature of 60°-200° C., to said joint for a period of time less than about 20 minutes. 
     
     
       8. A method according to claim 7 wherein said period of time is about 1-180 seconds. 
     
     
       9. A method according to claim 8 wherein the tensile shear strength of said joint approaches 4,000 p.s.i. over a period of about 30 days. 
     
     
       10. A method according to claim 7 wherein the pressure applied to said joint ranges from 20 to 2,000 p.s.i. 
     
     
       11. A method according to claim 6 wherein said step (c) and said step (d) are carried out simultaneously. 
     
     
       12. A method according to claim 6, wherein: a. said epoxide prepolymer and said polyamide are blended together in the presence of an organic liquid cosolvent for said epoxide prepolymer and said polyamide, whereby an organic liquid containing said epoxide prepolymer and said polyamide in solution is obtained; said organic liquid being coated onto a said substrate to form a wet film;   b. permitting said wet film to evaporate said cosolvent until a single, solid, latent-curable phase comprising an intermolecular mixture of said epoxide prepolymer and said polyamide is obtained;   c. applying heat and pressure to said joint, whereby said solid phase is liquefied and rendered non-latent in its curing characteristics; and   d. cooling said joint to normal ambient conditions without interrupting said amine/epoxide curing reaction.   
     
     
       13. A method according to claim 12 wherein said organic liquid is coated onto a series of substrates being continuously conveyed along an assembly line, wherein a series of joints is formed by mating pairs of substrates on said assembly line, and wherein each said joint is treated with heat and pressure to provide an adhesively bonded joint capable of withstanding further operations of said assembly line. 
     
     
       14. A method according to claim 6 wherein said epoxide prepolymer is a particulate solid uniformly dispersed through said uniform mixture. 
     
     
       15. A method according to claim 6 wherein said polyamide is a reaction product of the components comprising: a. a 1,4-bis-primary amino lower alkyl piperazine;   b. a polyfunctional carboxylic acid or functional equivalent thereof.   
     
     
       16. A method according to claim 15 wherein said uniform mixture further comprises at least additional ingredients selected from the group consisting of: a vinyl polymer, an epoxy-novolac resin, and a tackifier resin. 
     
     
       17. A method according to claim 6 wherein solvent for said polyamide is an organic liquid selected from the group consisting of: a lower alkanol, lower alkanoic amide, a chlorinated hydrocarbon, an aromatic compound, a nitrated hydrocarbon, a lower alkanoic acid, a lower alkanone, mixtures of said organic liquids with each other, and mixtures of any of said organic liquids with water. 
     
     
       18. A method according to claim 17 wherein said solvent is a cosolvent for said epoxide prepolymer. 
     
     
       19. A method according to claim 6 wherein the substrates to which said uniform mixture has been applied are stored after completion of said step (b) and kept in storage from 1 to 120 days before mating in accordance with said step (c). 
     
     
       20. A method according to claim 6 wherein said amine/epoxide curing reaction is carried out according to the following schedule:   ______________________________________                                    
Time        Temperature   Pressure                                        
______________________________________                                    
1 second -  60-200° C.                                             
                          20-2,000 p.s.i.                                 
20 minutes                                                                
12 hours -  normal ambient                                                
                          normal ambient                                  
30 days                                                                   
______________________________________                                    
     and whereby the following tensile shear strengths of said joint are obtained:     ______________________________________                                    
Time        Tensile Shear Strength in lbs/sq. inch                        
______________________________________                                    
1 second -  at least about 100                                            
20 minutes                                                                
12 hours -  from about 100 to 4,000                                       
30 days                                                                   
______________________________________